The present invention relates to a joining and bonding method and an apparatus for joining two members by using a DC current and/or a pulsated current and also to a unit of members joined by the same method, and in particular, to a joining method and an apparatus and also a unit of members joined by the same method, in which a plurality of members to be joined are placed in such a way that surfaces to be joined abut one another and a pulsated current and/or a DC current, in other words, at least either one of the pulsated current or the DC current is applied thereto so that the surfaces to be joined of these members are first joined temporarily, by using a technology such as pulse-excited sintering, which includes spark plasma sintering process (SPS), or a technology of hot-press joining, and thereafter heat-treating said temporarily joined to provide a permanent joint.
Various conventional methods have been used practically for joining a plurality of members desired. There are some well known methods including; (1) welding, in which areas to be joined are heated with a burner, an arc discharge or the like, and a weld-assisting material (i.e. a welding rod) having the same or similar material properties to the members to be joined is melted to be deposited on the members, thereby accomplishing joining; (2) brazing, which unlike welding uses a brazing filler metal (an alloy of Cu, Ag or the like, or an amorphous metal) consisting of a material different from members to be joined, which is melted between the joining surfaces of the members in an inactive atmosphere such as a vacuum atmosphere, nitrogen or argon atmosphere or the like, whereby those members are joined by the medium of said brazing filler metal; and (3) spot welding, in which relatively thin metal sheets are sandwiched between a pair of electrodes and electricity is applied therethrough, to cause welding.
Although the welding operation as described above in (1) includes a process for injecting an inactive gas such as nitrogen or argon to a heated portion to prevent it from being oxidized, since in this method a welding rod is used as the weld-assisting material to be deposited on the member (i.e. to form a padding) along an outer periphery of the joining surfaces, thereby allowing a plurality of members to be joined, such welding is not suitable for use when it is desired to join the entire surfaces of the members. In addition, welding has some defects including, (a) a weld-assisting member must be used; (b) a certain volume of a joining portion of a member must be cut off in advance to enable a welding rod melt to be deposited; (c) a welded portion has to be post-treated or processed, because it is impossible to deposit the melted substance from the welding rod precisely to a desired portion; (d) training is required to learn how to control a heating or welding temperature so that the metal composition of a member to be joined does not change; (e) training is also required to control a cooling temperature and rate so as not to cause a weld crack in the welded portion; (f) a resultant thermal deformation in the areas other than the joined portion and an increase in volume of the joined portion would occur inevitably; (g) the joining of members made of different materials is difficult; and (h) the joining is not applicable to a specific metal such as SUS, due to a change in the properties possibly caused by the welding.
Although brazing as described in (2) is advantageous in that it uses an intervening brazing filler metal placed over the entire joining surfaces, and joining may be accomplished at a temperature lower than that for the above described welding, it still involves some defects such as (i) a brazing filler metal must be used; (j) a joining strength obtained is no greater than the strength of the brazing filler metal; (k) there is an overflow of brazing filler metal upon brazing and thus finish processing must be carried out on the joined portion; (l) it is impossible to join the members in the desired areas precisely due to the overflow of brazing filler metal; (m) a joint uniform in appearance is difficult to attain; (n) strength of the joint will not be uniform especially in the outer periphery, from which the brazing filler metal is likely to flow out; and (o) joining of some metals such as stainless steel (SUS) is not feasible.
Further, although the spot welding described above in (3) can generates thermal energy between the pair of electrodes in a short time and allows narrow areas to be joined in a short time, it still has defects such that (p) only a member of a thin plate-like material can be joined; (q) post-welding distortion is great; (r) an extremely small area can be welded due to the occurrence of partial welding; and (s) post-welding deformation is considerable due to the occurrence of partial welding.
Still further, the above mentioned joining methods are applicable only to the joining process between metal members, and are not suitable for joining a metal member and a non-metal member, or non-metal members.
On the other hand, there has been another joining method suggested that takes advantage of the principle of hot-press joining using a DC current applied to the members to be joined under a predetermined pressure thus accomplishing joining; and there is known a spark plasma sintering process (SPS) which uses a DC pulsated current applied to members to thereby accomplish joining, but a joining method employing SPS technology is disadvantageous in that members to be joined are required to be placed within a die made of graphite and to be joined within said die, which makes the process too complicated. Further there are many restrictions on the shape of the members that can be joined, and, in addition, a sufficiently strong joint can not be obtained. There is known another joining method taking advantage of the principle of the spark plasma sintering process as disclosed in Japanese Patent Application Laid-open Publication No. Hei 11-158514. However, this method also suffers from some problems in that it is applicable only to members having rough surfaces, which entails pre-roughening surfaces for joining, which restrict its use, making it inapplicable, for example, to mirror-surfaces, and the like, and making it difficult to attain a sufficiently strong joint. Still further, even if the conventional hot-press joining process is applied to the joining of bulk members without any modifications, it is difficult to attain a sufficiently strong joint.
An object of the present invention is to provide a novel electric joining method and apparatus which can overcome the above-mentioned defects associated with the prior art while taking advantage of the technology of the pulse excited sintering process such as a spark plasma sintering process, and also to provide a unit of members joined by the same method and apparatus.
Another object of the present invention is to provide an electric joining method and apparatus which allows a rapid and rigid joining to be accomplished in joining areas by applying at least either one of a DC current or a pulsated current to a plurality of members to be joined to one another, and also to provide a unit of members joined by the same method and apparatus.
Still another object of the present invention is to provide an electric joining method and apparatus which allows an easy and rigid joining to be performed even between the members each made of different materials, and also to provide a unit of members joined by the same method and apparatus.
Still another object of the present invention is to provide an electric joining method and apparatus which makes possible not only joining of metal members but also joining between a metal member and a non-metal member or between non-metal members, and also to provide a unit of members joined by the same method and apparatus.
Still another object of the present invention is to provide an electric joining method and apparatus which allows the joining process to be applied relatively easily and reliably even to the joining between joining surfaces having the shapes different from each other, and also to provide a unit of members joined by the same method and apparatus.
Still another object of the present invention is to provide an electric joining method and apparatus which allows joining to be accomplished with minimal deformation after joining, and also to provide a unit of members joined by the same method and apparatus.
An electric joining method according to the present invention, comprises the steps of; placing the joining surfaces of a plurality of members to be joined so as to butt against one another, without using a die made of graphite; applying a pulsated current or a combination of DC current and a pulsated current to said members while applying a desired level of pressure to them so as to joint them temporarily; and performing a mutual solid-phase diffusion heat treatment for said temporarily joined members under a desired temperature condition.
In said electric joining method, said joining surfaces may be mirror-surfaces or rough surfaces. Further, said plurality of members may be made of substantially the same material or different materials. Still further, in said electric joining method, three or more pieces of said members to be joined may be disposed in series, so that they may be joined at a plurality of pairs of joining surfaces. Yet further, in said electric joining method, said pressure applied to said members may be not more than 50 Mega-pascal (MPa), said heat treatment may be conducted under inactive atmosphere, and said heat treatment may be conducted with the temperature controlled so as to equal to or lower than the temperature of 85% of the lowest melting point of the members to be joined. Besides a locating pin may be used in order to improve a locating accuracy of the joining surfaces.
Another invention disclosed in the present application provides an electric joining apparatus for joining a plurality of members to be joined to one another at respective joining surfaces thereof, said apparatus comprising: an electric joining device including a pair of energizing electrodes capable of contacting with said members so as to apply electricity thereto, a power supply connected to said pair of current-supply electrodes for supplying said energizing electrodes with a pulsated current or a combination of a DC current and a pulsated current, and a pressurizing unit for compressing each of said pair of electrodes against said joining surfaces, wherein said pair of members are sandwiched between said energizing electrodes and applied with at least either one of the DC current or the pulsated current from said power supply under a desired pressure, thus to be joined, without using a die made of graphite; and a heat treatment device for performing a mutual solid-phase diffusion heat treatment for the temporarily joined members under a desired atmosphere, and may still further comprises an inactive gas supplying source coupled to said heat treatment equipment for supplying inactive gas into said heat treatment equipment.
Still another invention disclosed in the present application provides a unit of joined members formed by joining a plurality of members using an electric joining method, wherein a heat treatment is applied to said plurality of members having been temporarily joined by applying electricity to said members.
Said unit of joined members may be composed of a plurality of members having their joining surfaces processed to be mirror-surfaces.